The invention relates to a method for the axial building of a hollow cylindrical soot body having no internal support and consisting substantially of silicon dioxide, in a vertical arrangement, by means of at least one flame hydrolysis build-up burner serving for the production of the soot, the soot being deposited on an auxiliary body at the beginning, and the build-up burner and the growing soot body being rotated relative to one another during the build-up, and the burner being maintained at an unvarying distance from the end of the hollow cylinder on which it is being built up.
A process of this kind is known from European Pat. No. A1-0,041,397 and corresponding U.S. Pat. No. 4,310,339. At the beginning of this process the soot is deposited on a suspended, rotating mandrel by means of burners. The soot body that is built up in this manner is continuously drawn downward axially in the course of the process, while an axial cavity is formed in the soot body due to the mandrel. If necessary, additional burners distributed axially can deposit additional soot layers on the soot body. Below the mandrel, at a slight distance from the latter, there is provided a heating means whereby the preform is consolidated. The consolidated preform is mounted in another turning means and continuously drawn downwardly.
German OS No. 26 47 121 and corresponding U.S. Pat. No. 3,966,446 disclose a process for the building up of a preform for optical fibers, in which the powdered substance is deposited by means of hydrolysis burners on a cylindrical mandrel revolving on a horizontal axis. The burner in this case is moved in several passages along the mandrel until a sufficient layer of soot has built up on the mandrel. In an additional system, the flame of the hydrolysis burner is horizontally aligned and deposits soot on a revolving output mandrel. The mandrel is continuously removed from the flame of the hydrolysis burner with a growing soot body. This process is limited to soot rods having a length of about 6 cm.
The flame hydrolysis technique is generally explained in U.S. Pat. Nos. 2,272,342 and 2,326,059. Reference may also be had to U.S. Pat. No. 3,868,170.
An object of the present invention is to provide a process for the build-up of elongated, hollow cylindrical soot bodies in which assisting or supporting bodies which might negatively affect the inside of the soot cylinder are avoided.